Integratable tunable resonant circuit for use in filters and oscillators

ABSTRACT

A circuitry arrangement for a resonant circuit that may be completely monolithically integrated and electrically tuned has a differential amplifier stage with two transistors T 1 ,T 1  &#39; fed by a constant current source I 0  that are differentially loaded at their collectors by a voltage-dependent capacity and inductively loaded in relation to the circuit ground by a pair of emitter followers T 2 ,T 2  &#39; with electrically adjustable impedance that act upon the base. To implement the electrically adjustable impedances, transistors with associated resistances are provided (for example a pair of emitter followers T 3 ,T 3  &#39; with base pre-resistances R 1 ,R 1  &#39;) which may be adjusted by control currents (for example I 1  I 1  &#39;) . The voltage-dependant capacity is implemented by transistors T 4 ,T 4  &#39; whose short-circuited emitters and collectors are connected so that the circuit node thus obtained is applied to the circuit ground through a voltage source U 1 . This circuit arrangement may also be used in a controllable oscillator by interconnecting two resonant circuits in a loop. The output signal of the first resonant circuit is used to control the second resonant circuit and the output signal of the second resonant circuit is inverted and used to control the first resonant circuit. Thus, two signals may be tapped at the outputs of both resonant circuits. The frequency of said signals may be varied by a control voltage and their frequency-independent phase difference equals π/2. An advantageous implementation of such an oscillator is obtained by decoupling the output signals of both resonant circuits through two identical output drive stages.

In IC technology, the aim is to design as far as possible all circuit components in a completely monolithically integrated fashion, in order to avoid the arrangement of external components. However, this encounters difficulties in the case of large capacitances/inductances, in particular, so that compromises or incomplete solutions are frequently accepted.

Thus, circuit concepts are known for monolithically integrated LC bandpass filters in which the necessary inductances are realized as planar spiral inductors by means of aluminum tracks on the chip. Such inductors are, however, not electrically adjustable, on the one hand, and they are only of low quality on the other hand, because of their series resistance and owing to parasitic coupling capacitances (N. M. Nguyen, R. G. Meyer: "Si IC-compatible inductors and LC passive filters", IEEE Journal of Solid-State Circuits, Vol. 25, No. 4, August 1990, pages 1028-1031; N. M. Nguyen, R. G. Meyer: "A Si bipolar monolithic RF bandpass amplifier", IEEE Journal of Solid-State Circuits, Vol. 27, No. 1, January 1992, pages 123-127).

The aim of the invention is to specify a completely monolithically integrated, electrically adjustable resonant circuit which can be used, for example, as an adjustable bandpass filter or as a resonator for a controllable oscillator and which satisfies all the requirements of high-quality IC technology. The invention achieves this aim by means of the basic concept in accordance with patent claim 1 and the development of this basic concept in accordance with patent claims 2-8.

The invention proceeds from the consideration of generating the required inductances by transformation of impedances from the base circuit into the emitter circuit of suitably connected bipolar transistors, the frequency dependence of the current gain being utilized. The inductances realized in this way on the one hand have a higher quality as an advantage with respect to the known concept and, on the other hand they can be electrically adjusted in a wide frequency range. Moreover, the geometrical dimensions of such inductors are smaller than those of the planar spiral inductors.

The utilization of the frequency dependence of the current gain of the bipolar transistors to realize inductances by transformation of impedance renders it possible to realize electrically tunable inductors by means of which the center frequency of a resonant circuit can be adjusted. As a second essential feature, the attenuation of the resonant circuit is reduced by negative resistances on the basis of the finite quality of the inductances thus generated, these negative resistances likewise being produced on the basis of impedance transformations. The change in filter capacities which is produced simultaneously with the tuning of the inductors leads to a relationship between the electrical manipulated variable and the center frequency of the resonant circuit which is approximately linear over a wide range.

The resonant circuit according to the invention can be used directly as an active single-stage or multistage bandpass filter. However, it is possible to use the resonant circuit according to the invention with particular advantage to form a completely monolithically integrated, electrically adjustable oscillator such as is required for many applications, for example in the field of telecommunications.

Various concepts are known for such oscillators and can be classified into the three groups of relaxation oscillators, ring oscillators and LC oscillators.

Relaxation oscillators (for example, M. Soyuer, J. D. Warnock: "Multigigahertz Voltage-Controlled Oscillators in Advanced Silicon Bipolar Technology", IEEE Journal of Solid-State Circuits, Vol. 27, No. 4, April 1992, pages 668-670; J. G. Sneep, C. J. M. Verhoeven: "A New Low-Noise 100-MHz Balanced Relaxation Oscillator", IEEE Journal of Solid-State Circuits, Vol. 25, No. 3, June 1990, pages 692-698; A. A. Abidi, R. G. Meyer: "Noise in Relaxation Oscillators", IEEE Journal of Solid-State Circuits, Vol. 18, No. 6, December 1983, pages 794-802), in which a capacitor is alternately charged and discharged via an adjustable current, certainly have a wide frequency adjustment range, but have high phase noise because of the lack of frequency selectivity of the amplitude response of the ring gain, as a result of which such oscillators are greatly restricted in the extent to which they can be used in many applications, for example in mobile radio or in the field of optical data transmission.

For the same reason, ring oscillators, in which the oscillation frequency is adjusted via a variation in the operating time of a plurality of invertor stages connected to form a ring, cannot be used for applications which necessitate an oscillator placing high demands on spectral purity (A. W. Buchwald, K. W. Martin: "High-Speed Voltage-Controlled Oscillator With Quadrature Outputs", Electronics Letters, Vol. 27, No. 4, 14 February 1991, pages 309-310).

Circuit concepts for integrated LC oscillators (N. M. Nguyen, R. G. Meyer: "A 1.8-GHz Monolithic LC Voltage-Controlled Oscillator", IEEE Journal of Solid-State Circuits, Vol. 27, No. 3, March 1992, pages 444-450) based on the abovementioned LC bandpass filters, in which the required inductors are realized as planar spiral inductors by aluminum tracks on the chip are better with reference to spectral purity. However, these have the disadvantage, in turn, that on the one hand they are not electrically adjustable and on the other hand are only of low quality, because of the series resistance as well as owing to the coupling capacitances.

By contrast, in accordance with the invention, a controllable, completely monolithically integrated oscillator is realized by a ring circuit consisting of two identical LC bandpass filters with electrically adjustable center frequencies. The advantage of this concept with respect to known circuit concepts resides in that the realization of electrically adjustable oscillators with low phase noise is possible without external resonators. Owing to the interconnection of the oscillator from two LC bandpass filters with adjustable center frequency, it is possible to tap at the two bandpass filter outputs two signals which have a constant phase difference of π/2 independent of frequency, as a result of which the circuit can be used as a quadrature oscillator. Additional advantages result from this for some applications. However, it can also suffice to output only one of the two output signals of the two bandpass filters.

Exemplary embodiments of the invention are explained in more detail below with the aid of the drawings, in which:

FIG. 1 shows in diagrammatic form the basic concept of a resonant circuit according to the invention and having adjustable impedances,

FIG. 2. shows a first example for realizing the adjustable impedances in the circuit in accordance with FIG. 1,

FIG. 3 shows a second example for realizing the adjustable impedances in the circuit in accordance with FIG. 1,

FIG. 4 an exemplary embodiment for an active bandpass filter, using the impedances realized in accordance with FIG. 2,

FIG. 5 shows an exemplary embodiment for an active bandpass filter, using the impedances realized in accordance with FIG. 3,

FIG. 6 shows the block diagram of an oscillator, formed with two bandpass filters in accordance with FIG. 2 or FIG. 3 and having quadrature outputs, and

FIG. 7 shows an exemplary embodiment of one of the driver stages included in the oscillator in accordance with FIG. 6.

The basic structure of the resonant circuit according to the invention is represented in FIG. 1. A differential amplifier stage consisting of the two transistors T₁ and T₁ ', which is fed by the constant source current I₀ has at the collectors of the said transistors an active load which consists of the transistors T₂,T₂ ' operated in a common collector circuit (emitter follower), as well as of the impedances Z₁, Z₁ '. For frequencies above the 3dB cut-off frequency of the current gain β (β cut-off frequency), this load represents an inductance with a series resistance to the extent that the impedances Z₁,Z₁ ', have a real part Re(Z₁) and Re(Z₁ '), respectively, differing from zero. For frequencies above the β cut-off frequency, the current gain β can be approximated by the relationship ##EQU1## β₀ designating the current gain at low frequencies, ω the angular frequency, ω.sub.β the 3 dB cut-off angular frequency of the current gain, and ω_(T) the transition angular frequency. The output resistance r_(A) of a bipolar transistor operated in a common collector circuit (emitter follower) with impedance Z_(B) acting on the base and a load Z_(E) connected to the emitter is given by ##EQU2## T_(C) designating the collector current and U_(T) the thermal voltage. In accordance with (1) and (2), the output impedances Z_(A2) and Z_(A2) of the transistors T₂ and T₂ ' can be described at their emitter terminals by series circuits each composed of a resistor R_(S) and an inductor L, the output impedances of the differential stage transistors T₁ and T₁ ' being neglected (Z_(E) →00, compare (2) ): ##EQU3##

The collector currents I_(C2) and I_(C2) ' of the transistors T₂ and T₂ ' amount on average to 0.5·I₀ and can be regarded as constant for small drive levels. The quality of the inductor can be optimized by suitable dimensioning of the imaginary parts of Z₁, Z₁ ' for the center angular frequency ω⁰ of the resonant circuit: ##EQU4##

The inductive load of the differential stage transistors produced in this way can be varied by adjusting the real parts of Z₁, Z₁ '. The filter capacitor C₁ required to realize a bandpass filter is connected differentially between the collectors of the differential stage transistors T₁,T₁ '.

As represented in FIG. 2, it is possible, for example, to use emitter followers T₃,T₃ ' with base bias resistors R₁,R₁ ' in order to realize the impedances Z₁, Z₁ ' with electrically adjustable real parts. The output impedance z_(A3) of the transistors T₃ and T₃ ' is calculated at their emitters by (1) and (2) to be: ##EQU5## the low loading of the transistors being negligible owing to the high input impedances of the transistors T₂,T₂ ' and to the likewise high output impedances of the current sources I₁,I₁ ', that is to say it is assumed that Z_(E) →oo (compare (2) ). The quality of the inductor can be optimized in accordance with (4) by suitable selection of the imaginary part of Z, that is to say by the dimensioning of R₁, R₁ ', for the center angular frequency of the resonant circuit, that is to say ω=ω₀ : ##EQU6##

The inductive load of the differential stage transistors T₁,T₁ ' produced in this way is inversely proportional to the collector current of the transistors T₂,T₃ '. The filter capacitor between the collectors of the differential stage transistors T₁,T₁ ' is realized by means of the base-collector- and the base-emitter-depletion layer capacitances of the transistors T₄,T₄ '.

Both the voltage dependence of the filter capacitance and the current dependence of the inductive load are used to detune the center frequency of the bandpass filter by varying U₁ and I₁, I₁ '. The bandpass output signal U_(Q) can be tapped at the collectors of the differential stage transistors T₁,T₁.

A second variant for realizing the impedances Z₁,Z₁ ' by means of electrically adjustable real parts is represented in FIG. 3. This variant is particularly advantageous in the case of low operating voltages such as, for example, in battery operation. The impedances Z₁, Z₁ ' are realized here by a circuit comprising the resistors R₈,R₈ ', the transistors T₈ and T₈ ', connected as diodes, the adjustable current sources I₁ and I₁ ', the resistor R_(1A), the transistors T_(N) and T_(N) ', the current sources I_(N) and I_(N) ' and the capacitor C₂. The impedances can be calculated as follows taking account of (1) and (2): ##EQU7##

The inductive load of the differential stage transistors T₁,T₁ ' generated in this way is a function of the collector current of the transistors T₈,T₈ '. The filter capacitance between the collectors of the differential stage transistors T₁,T₁ ' is, in turn, realized by means of the base-collector- and the base-emitter- depletion layer capacitances of the transistors T₄,T₄ '. The capacitance of the capacitor C₂ can be optimized for ω=ω₀ in order to deattenuate the bandpass filter.

As in the example of FIG. 2, here, as well, both the voltage dependence of the filter capacitance and the current dependence of the inductive load are used to detune the center frequency of the bandpass filter by varying U₁ and I₁, I₁ '. The bandpass output signal U_(Q) can be tapped at the base terminals of the emitter followers T₂, T₂ '.

An advantageous embodiment of a bandpass filter based on the basic structure realized in accordance with FIG. 2 is represented in FIG. 4. The bandpass filter comprises in essence the basic structure represented in FIG. 2, the adjustment of the current-dependent inductances and of the voltage-dependent capacitances being realized as a function of the control voltage U_(s) by means of a feedback differential amplifier stage which consists of the transistors T_(5A),T_(5B),T_(5A) ',T_(5B) ' and the resistors R₄,R₄ ' and which is fed by the constant current source I₂. Owing to the additional constant currents I₃,I₃ ', the collector currents I_(C2) and I_(C2) ' of the transistors T₂ and T₂ ' amount on average to 0.5·I₀ +I₃ and 0.5·I₀ +I₃ ', respectively. With I₃ =I₃ '>>I₀, the inductances become independent of the drive level of the feedback differential amplifier stage comprising T₁,T₁ ' and R₂,R₂ '.

For level adaptation and decoupling, the bandpass output signal u_(Q) is tapped at the collectors of the differential stage transistors T₁,T₁ ' via the emitter followers T₆,T₆ ', which are fed from the constant current sources I₄, I₄ '.

An embodiment of a bandpass filter based on the basic structure realized in accordance with FIG. 3 which is particularly advantageous for low operating voltages is represented in FIG. 5. Just as in the case of the exemplary embodiment in accordance with FIG. 4, here the adjustment of the current-dependent inductances and of the voltage-dependent capacitances is realized as a function of the control voltage U_(s) by means of a feedback differential amplifier stage which comprises the transistors T_(5A),T_(5B),T_(5A) ',T_(5B) ' and the resistors R₄,R₄ ' and is fed by the constant current source I₂.

The additional constant currents I₃,I₃ ' have the same effect as in the exemplary embodiment in accordance with FIG. 4. For the purpose of level adaptation and decoupling, the bandpass output signal U_(Q) is tapped at the base terminals of the load transistors T₂, T₂ ' via the emitter followers T₆,T₆ ', which are fed from the constant current sources I₄, I₄ '.

The circuits in accordance with FIG. 4 and FIG. 5 can be used both as single-stage adjustable bandpass filters and, by cascading a plurality of stages, as multistage adjustable bandpass filters, it being possible to additionally enhance the quality of the multistage bandpass filters by mutually detuning the center frequencies of the cascaded filter stages (so-called "staggered tuning").

In the exemplary embodiment in FIG. 6, two active bandpass filters 1, 2 having differential inputs and outputs are connected to form a ring in such a way that the active bandpass filter 2 is driven by the output signal of the active bandpass filter 1, the non-inverting output Q of the active bandpass filter 1 driving the non-inverting input I of the active bandpass filter 2, and the inverting output Q of the active bandpass filter 1 driving the inverting input I of the active bandpass filter 2. By contrast, the active bandpass filter 1 is driven in an inverted fashion by the output signal of the bandpass filter 2 owing to the fact that the non-inverting output Q of the active bandpass filter 2 is connected to the inverting input I of the active bandpass filter 1, and the inverting output Q of the active bandpass filter 2 is connected to the non-inverting input I of the active bandpass filter 1.

In the case of the resonant frequency, the bandpass filter output signals are phase-shifted with respect to the respective bandpass filter input signals by -π/2. Owing to the inversion in the case of driving the active bandpass filter 1 by the output signal of the active bandpass filter 2, which corresponds to an additional phase rotation of -π, the phase in the ring circuit is rotated overall by 2 π, that is to say the positive feedback required for the oscillation is achieved. The amplitude condition required over and above this is fulfilled owing to the fact that the gain of the active bandpass filter is greater than one.

In accordance with FIG. 6, the output signals are advantageously tapped via two output drivers 3, 4. As a result, the load capacity of the oscillator is increased and decoupling of the load to be driven is achieved. An exemplary embodiment for suitable driver stages is represented in FIG. 7. Together with the negative-feedback resistors R₅,R₅ ', the transistors T₇,T₇ ' form a differential stage which is loaded by the load resistors R₆, R₆ ', the output signals u_(Q1) and u_(Q2) of the bandpass filters being used as driving signals of the driver stages. If no quadrature output signals are required, the use of an individual output driver which is driven by one of the two bandpass filters is sufficient. 

I claim:
 1. Monolithically integrated, tunable resonant circuit containing a differential amplifier stage having two transistors T₁,T₁ ' which are fed from a constant current source I₀ and are loaded inductively with respect to the circuit ground at their collectors by means of an emitter follower pair T₂,T₂ ' having electrically adjustable impedances Z₁,Z₁ ' which act on a base of the transistors T₂,T₂ ' and simulate inductances by impedance transformation via the transistors T₂,T₂ ' wherein impedance Z₁ acts on the base of T₂ and impedance Z₁ ' acts on the base of T₂ ', a voltage-dependent capacitor C₁ being connected between the collectors of the differential stage transistors T₁,T₁ '.
 2. Resonant circuit according to claim 1, characterized in that the impedance Z₁,Z₁ ' are simulated by emitter followers T₃,T₃ ' which are provided with base bias resistors R₁,R₁ ' and the emitter followers T₃,T₃ ' can be adjusted via control currents I₁, I₁ ' applied to the emitter terminals of the emitter followers T₃,T₃ '.
 3. Resonant circuit according to claim 1, characterized in that the impedances Z₁,Z₁ ' are simulated by two transistors T₈,T₈ ' which are connected as diodes and have assigned resistors R₈,R₈ ' wherein the internal resistance of resistors R₈, R₈ ' can be adjusted via control currents I₁,I₁ ', as well as by two further transistors T_(N),T_(N) ' which can be adjusted via control currents I_(N),I_(N) ' and are situated between the bases of the emitter follower pair T₂,T₂ ' the emitter terminals of the transistors T₈,T₈ ' being interconnected by a resistor R_(1A) and the emitter terminals of the transistors T_(N),T_(N) ' being interconnected by a capacitor C₂.
 4. Resonant circuit according to claim 1, characterized in that the voltage-dependent capacitance C₁ is formed by two transistors T₄,T₄ ' whose respectively short-circuited emitters and collectors are connected at a circuit node, and are connected by means of this circuit node to the circuit ground via a voltage source u₁.
 5. Resonant circuit according to claim 1, characterized in that the linearity range of an input voltage u_(I) of the differential amplifier stage with the transistors T₁,T₁ ' is increased by two negative-feedback resistors R₂,R₂ '.
 6. Resonant circuit according to claim 1 characterized in that a pair of control currents I₁,I₁ ' are generated from a constant-current source I₂ by means of a differential amplifier stage having four transistors T_(5A),T_(5B),T_(5A) ',T_(5B) ' and two negative-feedback resistors R₄,R₄ ' as a function of a control voltage U_(s), a voltage source U₁ being formed by a resistor R₃ at which a portion of the source current I₂ effects the voltage drop U₁ which serves to control voltage-dependent capacitances T₄,T₄ ' which are connected at their respective bases to the emitter of T₂ and the emitter of T₂ '.
 7. Resonant circuit according claim 1, characterized in that an output signal u_(Q) is tapped differentially via respective emitter terminals of an emitter follower pair composed of two transistors T₆,T₆ ' which are connected by constant current sources I₄,I₄ ' and which serve the purpose of decoupling and level reduction, T₆ being fed by I₄ and T₆ ' being fed by I₄ ', said emitter follower pair including bases which are respectively connected to the collectors of the two transistors T₁,T₁ ' and collectors which are connected to ground.
 8. Resonant circuit according to claim 1, characterized in that the emitter followers T₂,T₂ ' are additionally fed with constant currents I₃,I₃ '.
 9. Circuit arrangement according to claim 1, characterized by using ECL technology.
 10. Multistage bandpass filter consisting of a plurality of cascaded resonant circuits in accordance with claim
 1. 11. Circuit arrangement according to claim 1, characterized by using E² CL technology.
 12. Controllable oscillator consisting of two resonant circuits in accordance with claim 1, and wherein the two resonant circuits are connected to form a ring, the second resonant circuit having an output signal u_(Q2) used in an inverted fashion to drive the first resonant circuit having an output signal u_(Q1).
 13. Oscillator according to claim 12, characterized in that the output signals u_(Q1) and u_(Q2) of the two resonant circuits (1) and (2) are decoupled by two identical output driver stages (3) and (4), each of these two output driver stages including a differential amplifier stage having two transistors T₇,T₇ ' which are driven in a differential fashion between the base terminals by means of the signals u_(Q1) and U_(Q2), the differential amplifier stage being fed by a constant current source I₅, and the output signal u'_(Q) being tapped in a differential fashion between a pair of load resistors R₆,R₆ ' of the differential amplifier stages in each of the output driver stages.
 14. Oscillator according to claim 13, characterized in that the linearity range of an input voltage u_(I), of the differential amplifier stage having the transistors T₇,T₇ ' is increased by two negative-feedback resistors R₅,R₅ '.
 15. Oscillator according to claim 12, characterized in that respective output signals u_(Q) of the first and second resonant circuits, or respective output signals U_(Q) ' of the output driver stages have a constant phase difference of π/2 independent from frequency.
 16. Oscillator according to claim 12, characterized in that only one of said u_(Q1) and u_(Q2) output signals of said respective first and second resonant circuits is output.
 17. Oscillator according to claim 12, characterized in that the output signals u_(Q1) and u_(Q2) of the two resonant circuits (1) and (2) are decoupled by two identical output driver stages (3) and (4), each of these two output driver stages including a differential amplifier stage having two transistors T₇,T₇ ' which are driven in a differential fashion between the base terminals by means of the signals u_(Q1) and u_(Q2), the differential amplifier stage being fed by a constant current source I₅, and the output signal being tapped in a single-phase fashion at resistors R₆ and R₆ ' respectively, each relative to ground. 